As it is known, the disc brake comprises a disc solid to the wheel and a caliper mounted on a bracket engaged to the yoke of the motorcycle or directly on the frame and/or on the fork.
The caliper is provided with small pistons which exert a braking action on the disc by operating, for example, a pedal pump mounted in proximity of one of the motorcycle footboards.
The pump comprises a piston sliding and/or rotating within a cylinder intended to hold fluid and moved, via suitable linkages, by the driver-operated pedal.
The cylinder has an opening at a longitudinal end thereof, opposite to the piston and connected, by means of a suitable hydraulic connection, to the caliper, or servo actuator, of the rear brake. In the known calipers, the pump is further provided with a compensating reservoir exhibiting a conduit which leads on a side wall of the cylinder into two holes being in side-by-side relationship, a feeding hole and a compensating hole. The piston has two gaskets tight-seal sliding on the inner wall of the cylinder, and a central portion interposed between said gaskets and delimiting an annular compensating chamber or space. The feeding hole—when the fluid is under pressure during the motion of the piston—leads always into the compensating space, while the compensating hole opens always onto and between the ends delimiting the gaskets. The feeding hole has the function—besides allowing the expansion of the fluid in case the latter is heated up—also of recirculating a greater amount of brake fluid, as required by the wear of the pads. In fact, in the absence of the feeding hole, the pump's piston would advance upon every braking action by a minimum additional extent owing both to the wear of the pads and the exceeding stroke length that the pistons of the calipers have to perform. Instead, with the compensating mechanism, upon releasing the pressurizing control, the caliper's pistons move back by approximately the same extent (“roll back” effect of the caliper piston's gaskets) and the piston of the pump moves back to a position which does not coincide with the previous one. However, the increase in volume between the piston's main gasket and the pistons of the calipers is compensated through the feeding hole.
The volume of fluid necessary for the compensation is therefore taken usually from the reservoir through the feeding hole and, only in case of anomalous wear or knock off, from the intermediate compensating chamber between the two gaskets of the pump's piston with a lowering of the lip of the main gasket and the withdrawal of fluid from the compensating reservoir through the compensating holes disposed on the pump's float.
The Applicant has however found out that the structure of the current compensating systems is bulky and weighs heavily on the overall dimensions and weight of the pumps of known type.
Since in the field, for example, of motorcycles and bicycles—and in general in all cases where highest performance levels are sought—the current trend is to reduce weights and dimensions in order to increase the handling and performance of the means in question, the weight and dimensions of a component like the brake pump are also of great consideration.
In particular, the presence of the two holes lined up along the cylinder of the pump, and the presence of the compensating chamber located between said gaskets, is cause for a sizeable axial development of the brake pumps of known type.
A further drawback of the known systems is the limited duration of the main gasket, due to the fact that the feeding hole may have small rugs left by the manufacturing process, which require costly machining operations to remove them and, if not removed, may be cause of an early wear out the gasket, especially under operating conditions. Moreover, in the first phase of compression of the fluid held in the main chamber, the main gasket must close the feeding hole quickly, so as to prevent a portion of the fluid from returning into the reservoir and thereby nullifying the effect of part of the working stroke. Such closing is, in the known pumps, committed to the axial sliding of the piston which is solid to said main gasket, and takes place with some delay after the actuation of the pump, also according to the idle stroke.